Water levels are of course determined by means of floating switches, ultrasonics, electrodes or pressure monitors, the pump being switched on and off accordingly. However, the known devices each suffer from at least two of the following problems: nonadjustability, inaccuracy, expensive construction, high cost, technical complication, and significant unreliability.
The unreliability of floating switches is due to the fact that solids may become deposited thereon, electrodes may become dirty; in ultrasonics the echo may suffer interference from foam on the water surface. The mechanical apparatus of pressure monitors is vulnerable.